! Project Summary/Abstract Children with language-based learning disabilities, including specific language impairment (SLI) and dyslexia, comprise over 57% of children receiving special education services in public schools (> 3 million children). Although they are separate disorders, SLI and dyslexia frequently co-occur in the same individuals. Difficulty learning new words is considered a central feature of SLI, whereas for children with dyslexia, vocabulary deficits are typically viewed as a byproduct, rather than a cause, of reading problems. However, most studies of word learning in children with SLI and dyslexia have involved heterogeneous groups, which likely contain many cases of comorbidity. Our research team has been the first to study word learning in children with SLI who have good word reading skills (SLI-only) and children with dyslexia who have good oral language skills (dyslexia-only) as compared to children with both SLI and dyslexia (SLI+DYS) and peers with typical development (TD). Our convincing preliminary results suggest a paradox between experimental word learning studies and standardized measures of vocabulary size. First, despite performing comparably to TD peers on standardized measures of vocabulary size, children with dyslexia-only have considerable difficulty learning novel words in experimental word-learning paradigms. Second, children with SLI-only performed significantly better than those with dyslexia-only despite scoring significantly lower than those with dyslexia-only on standardized measures of vocabulary size. Our long-term goal is to better understand the paradox between experimental word-learning studies and standardized measures of vocabulary size and to inform clinical practice with regard to the assessment and treatment of word-learning deficits as related to literacy outcomes. In this project, we will test theory-driven, clinically relevant hypotheses about word learning and its relation to literacy outcomes in a longitudinal study of children with SLI, dyslexia or both (SLI+DYS) compared to their TD peers, from 2nd to 4th grade. Specific aims are: (1) to test three theory-driven factors that may explain SLI and dyslexia subgroup differences in word-learning at 2nd grade: quantity of exposures, type of instruction, and stages of word learning; (2) to determine the contribution of linguistic, domain-general, and home environment measures to explain individual differences in 2nd grade word learning; and (3) to predict literacy outcomes in 4th grade from measures of word learning in 2nd grade. These aims fill theoretical gaps regarding the mechanisms for word-learning deficits in children with SLI and dyslexia and address methodological limitations of past studies by considering comorbidity and including carefully selected groups of children with a range of language and literacy abilities. Important innovations include connecting disparate lines of inquiry (experimental word learning and academic outcomes; SLI and dyslexia), contrasting theoretical factors with relevance to clinical decision-making (dosage, type of instruction, learning stage), and using an ecologically valid incidental word- learning paradigm that reflects how children learn words from electronic media. !